Okay, so imagine if you had a group of people, let's call them Tom, Jerry, and Billy. Now, let's say you're in charge of keeping them in line and making sure they don't do anything wrong. That's kind of like what the NKVD was in charge of in the 1930s and 1940s in the Soviet Union.
Now let's say one day you get a report that Tom, Jerry, and Billy might be up to something bad. Maybe they're plotting to do something that could hurt other people. You would want to investigate that, right? And if you found out they were really going to do something bad, you might have to take action to stop them.
That's kind of what the NKVD did, except on a much bigger scale. They were in charge of investigating people who might be doing things that were against the government or against the Soviet Union. This could include anything from talking bad about the government to actually trying to overthrow it.
When they found someone who they thought was doing something bad, they would often take them away in the middle of the night and take them to prison or some other place where they could be questioned. This was often called a "mass operation" because they would go after a large group of people all at once.
Unfortunately, sometimes the people they took away were actually innocent. Maybe they were just in the wrong place at the wrong time, or maybe someone had given them bad information. This was one of the problems with the mass operations of the NKVD – innocent people could be hurt or killed.
So, in summary, the NKVD was like a group of people who were in charge of keeping everyone in line in the Soviet Union. When they thought someone was doing something bad, they would investigate and maybe take them away to question them. This was often called a "mass operation" because they would go after a large group of people all at once. But sometimes innocent people were hurt, which was a big problem.